Since many of you post over and over about this error, here is a post(hopefully made into a stickey)! w00t go me.


Troubleshooting Persistent Steam UserID Ticket Errors
If you have determined that the Steam network is not down and you are still receiving this error message, this issue is most likely being caused by not restarting after a Steam update has been released.

If restarting Steam does not resolve the issue, please exit Steam and delete the following files:

Steam.dll
SteamUI.dll
ClientRegistry.Blob
These will be found under your Steam installation directory - by default, this will be C:\Program Files\Valve\Steam\ (this may vary if you specified a different installation folder, or if you have moved your Steam installation).